Originally posted by: rgwalt
Originally posted by: Paratus
What interesting is that as the viruses reproduce and go through different generations they inevetiably evolve into less lethal forms. Look at the Flu - killed 20 million in the early 20th century, now its only dangerous the very old or young. Both humans and virii adapt to each other.
I think that has more to do with better medical technology than anything else.
Actually, it probably isn't completely.
Prior to the major 1918 global pandemic, 'flu was a relatively minor illness. Yes, it was dangerous to the infirm, elderly and young, but death was unsual (estimated at probably less than 1 in 1000).
Something was different about the 1918 virus. It was much more contagious and much more virulent. It would rapidly infect young fit people, and they would develop pneumonia and die (death rate probably 1 in 40).
This was probably not so much due to a significant change in the virulence of the virus, but instead because it had 'changed its spots'. Flu viruses endemic in a population don't generally cause much illness because the population is immune. Slow changes in the virus mean that the population can retain some immunity against slightly different viruses. The big pandemics are thought to have occurred because the virus simply picked up a new coat from another virus (e.g. from swine flu or bird flu), to which the population have no immunity at all.
Finally, it's not always that case that pathogens continually evolve into less lethal forms - it depends on the mode of operation of the pathogen and the populations. The classical example is HIV. HIV has a very rapid rate of genetic change, and can evolve rapidly. The HIV infecting the general population in Africa is considerably less virulent than believed to be at discovery. Conversely the HIV infecting drug abusers and gay men in Western inner cities, is extremely virulent and causes rapid progression to AIDS.
The difference is in the methods of spread. In Africa, spread is sexual primarily sexual, and as many relationships are stable and monogamous, a virus that replicates quickly inside its host will fail to spread. Conversely, drug abusers may share needles with a huge number of people over a period of months, so an aggressive virus which rapidly overwhelms its host does not lose its transmission ability.
I think you are right......look at the plague of the 16th century......killed 10's of millions across europe
Actually, this disease (plague) is a highly dangerous disease today, although it can be treated with penicillin. The bug that causes it (Yersinia pestis) is highly virulent and can cause very severe pneumonia as well as causing septicaemia. Thankfully, better hygeine and removal of its vector, the flea, and its host, the black rat, has greatly reduced the incidence of this disease.